Northwest Is Shakin’ Two Temblors, Following Monday’s 4.9 Rumble, Are Reminder Of Region’s Seismic Proclivity

Just a coincidence, but also a sober reminder the Pacific Northwest is prime earthquake country.

That’s how scientists characterized the three moderate earthquakes that rattled different parts of Washington state in less than 24 hours.

The quakes, all greater than magnitude 4.0 and felt widely, caused no serious damage. All occurred on different fault lines and were unrelated, said Tom Yelin, a U.S. Geological Survey geophysicist in Seattle.

“Occasionally when you roll dice, you come up with three snake eyes in a row,” he said. “It’s not terribly likely, but it does happen. I think that’s the case here.”

Two magnitude 4.6 quakes struck within minutes of each other - but nearly 200 miles apart - Tuesday morning, rousing many residents from their sleep. They came a day after a 4.9 temblor shook the Seattle and Puget Sound area.

The first 4.6 temblor struck at 7:23 a.m., centered about 14 miles west of the northcentral Washington farm town of Okanogan. It was felt across north-central Washington and in the Spokane area, and as far north as Kelowna and Vernon in British Columbia. A 3.6 aftershock hit minutes later, said Bill Steele, coordinator of the University of Washington’s seismology laboratory in Seattle.

Emergency management officials were checking dams and other structures as a precaution, but no major damage was reported, Okanogan County Sheriff Jim Weed said.

“It did sound like a big rumbling, like when we have an apple truck loaded with apple bins, when they come down the grade, or a cattle truck makes that sort of a rumbling noise,” said Marilynn Moses, coordinator at the Okanogan County Historical Society museum in Okanogan. “But this was much louder and longer, lot of vibration, mirrors rattling on the wall. It was different.”

Betty Hopper, a retired widow who lives alone in a double-wide mobile home about 5 miles south of Okanogan, was standing in her bedroom when things started rattling.

“It shook like crazy. Two things fell off the top shelf - a glass vase and some stuffed animals - and it scared the heck out of me,” she said.

“I panicked and I thought, ‘Which way do I run?”’ At 7:41 a.m., a magnitude 4.6 quake struck just east of Vancouver Island, or about 25 miles west-northwest of Vancouver, British Columbia.

It was felt by residents in the Vancouver area and on Vancouver Island, as well as Washington’s Olympic Peninsula and Whatcom County.

Bob Breen of suburban Richmond, British Columbia, was asleep when the quake hit. “All of a sudden the bed started to move and it woke me and the bed probably moved for about eight to 12 seconds,” he said.

Tuesday’s quakes followed a magnitude 4.9 temblor that struck the Puget Sound area shortly after noon Monday.

Monday’s quake, centered about 3.7 miles northeast of Bremerton, or about 13 miles west of Seattle, knocked a trailer in Silverdale slightly off its foundation and opened minor cracks on seven buildings at the Puget Sound Naval Shipyard in Bremerton. It was followed by several aftershocks.

“It was mostly insignificant damage,” said Donald Pratt, Bremerton’s community development director. “In most cases, there was a structural problem to begin with that the quake kind of called attention to.”

“Why we’re having three greater than magnitude 4 quakes on different faults and in different parts of the state is a coincidence,” Steele said.

All three quakes were “shallow” quakes, or ones that occur within a few miles from the surface.

Scientists have long had evidence of the dangers of two other types of quakes in the region: massive, magnitude 9 “subduction” quakes, caused by contact between the bottom of the continent and the top of shifting ocean rocks, and “deep” quakes of magnitude 6.5 to 7.5 beneath central Puget Sound, where the shifting ocean rocks push beneath the North American continent.

But shallow quakes weren’t considered much of a threat in the Northwest until the early 1990s, with the discovery of the Seattle Fault running underneath the city near downtown and major ports. That fault is believed to be the source of a devastating quake 1,100 years ago that jacked up land masses, spawned a huge tidal wave and sent forests sliding to the bottom of Lake Washington.

Monday’s quake is believed to have occurred on a subsidiary of the Seattle Fault.

Although hundreds of quakes occur in the Northwest region of Oregon, Washington and southern British Columbia each year, only one to two dozen are felt. So far this year, the UW seismology lab has received 15 reports of quakes felt in the region.

Monday’s 4.9 quake was the strongest in the area since a 5.4 magnitude temblor near Duvall, northeast of Seattle, in May 1996.

The last major quakes to cause substantial damage in Washington were a 7.1 shock near Olympia in 1949 and a 6.5 quake between Seattle and Tacoma in 1965. Those quakes killed a total of 15 people and caused more than $200 million in property damage.

This week, USGS scientists also began a 10-day marine-seismic survey aboard a small boat, using sound waves to map faults that lie beneath the Puget Sound lowlands. The data collected will be factored into “seismic hazard maps.”